The present invention relates to a method and system of supplying hydrogen for use in a fuel cell. The system and method produces hydrogen by a reforming reaction of a hydrocarbon stream, and is particularly useful for supplying hydrogen to vehicles and stationary structures that use fuel cells.
Recently there have been efforts to develop systems for supplying hydrogen to fuel cells that are used in such applications as vehicles, such as cars and buses, or stationary structures such as industrial plants. One such type of system that has been proposed obtains hydrogen from a reforming reaction of hydrocarbons or oxygen containing hydrocarbons. The hydrogen produced is purified in a hydrogen membrane separator before being used in the fuel cell.
The major reactions that occur in reforming can be represented by the following equations:
CnH2(n+1)+nH2Oxe2x86x92←nCO+(2n+1)H2(Endothermic)xe2x80x83xe2x80x83Equation (I)
xe2x80x83CnH2(n+1)+2nH2Oxe2x86x92←nCO2+(3n+1)H2(Endothermic)xe2x80x83xe2x80x83Equation (II)
CO+H2Oxe2x86x92←CO2+H2(xe2x88x9241 kJ/mole)xe2x80x83xe2x80x83Equation (III)
The reactions are equilibrium reactions and therefore, the amount of hydrogen produced from the hydrocarbon depends upon the reactions conditions, such as concentration of reactants, temperature, and pressure. For example, high concentrations of carbon dioxide consumes hydrogen to produce hydrocarbons such as methane (Equation II) and carbon monoxide (Equation III). However, increasing the amount of water drives the reactions to produce hydrogen. Therefore, efforts have focused on maximizing hydrogen production in these equilibrium reactions. Also, the reactions as written (from left to right) of Equations I and II are highly endothermic, requiring heat to drive the reactions, while the water gas shift reaction of Equation III is only slightly exothermic. Thus another effort has been to find efficient ways of supplying heat to the reforming reaction.
Once the hydrogen is produced, it typically is purified to remove such by-products as carbon monoxide to prevent poisoning of the catalyst coated electrodes (such as platinum coated electrodes) in the fuel cell. This purification may be performed using a hydrogen membrane separator. The membrane is typically a film or material that selectively allows hydrogen to pass through. The inlet side of the membrane, hereinafter called the xe2x80x9cretentate sidexe2x80x9d is typically at a higher pressure than the outlet side, hereinafter called the xe2x80x9cpermeate side.xe2x80x9d The pressure difference between the permeate side and retentate side helps to drive the separation of the hydrogen. Suitable membranes include for example thin tubes or foils of palladium and alloys of palladium with silver or copper. The purified hydrogen leaving the membrane (hereinafter called the xe2x80x9chydrogen permeatexe2x80x9d is fed to the fuel cell, while the material that did not pass through the membrane, hereinafter xe2x80x9cretentatexe2x80x9d is combusted to provide process heat.
There are various types of fuel cells that use hydrogen including for example alkaline fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. For fuel cells used in vehicles, polymer electrolyte fuel cells are most preferred.
In a polymer electrolyte fuel cell, purified hydrogen is fed to an anode side of the fuel cell where the hydrogen is split to form two hydrogen ions and two electrons. The hydrogen ions travel from the anode to the cathode by passing through a hydrated solid electrolyte that is continuously moistened with water. The electrons pass from the anode to the cathode by passing through an external circuit to supply electrical power. At the cathode, the hydrogen ions and electrons are reacted with oxygen in the air to form a fuel cell exhaust stream containing water vapor and oxygen depleted air.
The systems thus far proposed for supplying hydrogen to a fuel cell have been sub-optimal. For example, one problem has been finding efficient ways to supply heat for starting-up and maintaining the reforming reaction which is endothermic. U.S. Pat. No. 5,741,474 to Isomura et at., (hereinafter xe2x80x9cIsomuraxe2x80x9d) and U.S. Pat. No. 5,746,985 to Takahashi, (hereinafter xe2x80x9cTakahashixe2x80x9d) disclose systems for producing high purity hydrogen by reforming a hydrocarbon and/or an oxygen atom-containing hydrocarbon in the presence of steam to form a reformed gas containing hydrogen. The reformed gas is passed through a hydrogen membrane separator to be purified. To provide heat for the reforming reaction, Isomura and Takahashi teach that oxygen or air can be fed to the reforming reaction, in addition to the steam and hydrocarbon source, to carry out a partial oxidation reaction simultaneously with the reforming reaction. The partial oxidation is exothermic and supplies heat to maintain the reforming reaction. Isomura additionally teaches that the retentate from the hydrogen membrane separator can be combusted to supply heat for heating and vaporizing the reactants. Takahashi teaches that electrical resistors can be embedded in the catalyst to assist in starting-up and maintaining the reforming reaction. However, the systems of Isomura and Takahashi are sub-optimal in that gases formed or present during the partial oxidation reaction, such as nitrogen from the air, dilute the reformate, thereby reducing the effectiveness of the hydrogen recovery.
Additionally, the products produced, in addition to the hydrogen, have not been efficiently utilized in the system. For example, in Isomura, the steam-oxygen depleted gas mixture emitted from the fuel cell is condensed to remove the water. The oxygen depleted gas is simply discharged, while the water is re-circulated to moisten the purified hydrogen entering the fuel cell. Although the products from the fuel cell and their energy are partially used, more efficient uses of the products could be made.
U.S. Pat. No. 5,686,196 to Singh et al. (hereinafter xe2x80x9cSinghxe2x80x9d) discloses a system for operating a solid oxide fuel cell generator using diesel fuel. The reformer produces hydrogen from the reforming reaction of desulfurized diesel fuel. The hydrogen produced is separated from the other reforming reaction products and is sent to a hydrogen storage device or is mixed with the diesel fuel prior to desulfurization. The remaining reaction products from the reforming reaction are fed to a solid oxide fuel cell where water generated from the operation of the fuel cell is recycled back to the reforming reactor. The system in Singh, although recycling some streams, is also sub-optimal in that the reactant by-products produced in the system could be used more efficiently to supply heat and energy to other system members.
Thus, there is a need in the art for a method and system, based on the reforming of hydrocarbons, for efficiently supplying hydrogen to a fuel cell. Particularly, there is a need to more efficiently use the products, heat, or energy generated in the reforming reaction and fuel cell to operate the system. There is also a need to optimize the yields of hydrogen obtained from the reforming reaction while maintaining energy efficiency of the system. There is also a need in the art for an improved method of starting-up the reforming reaction. The present invention seeks to solve these and other needs in the art.
The present invention provides an efficient method and system, based on the reforming of hydrocarbons, for producing hydrogen for use in a fuel cell system. The method and system of the present invention uses the products and the associated energy and/or heat produced from the system to operate the system. The method of the present invention includes feeding a hydrocarbon stream and water stream into a reforming reaction zone where the hydrocarbon stream and water stream are vaporized prior to or upon entering the reforming reaction zone of a reactor having a reforming catalyst; reacting the vaporized hydrocarbon stream and water stream in the reforming reaction zone at a temperature of at least about 200xc2x0 C. and a pressure of at least 100 kPa to produce a gaseous reformate stream containing hydrogen; and feeding the gaseous reformate stream into a hydrogen separating membrane to form a purified hydrogen stream and a retentate stream. The method also includes forming a retentate recycle stream and an exhaust tail gas stream from the retentate stream in proportions to provide a retentate recycle ratio of about 20:1 to about 1:20; and recycling the retentate recycle stream to the reforming reaction zone and directing the exhaust tail gas stream to a combustor. The method further includes oxidizing the exhaust tail gas stream in the combustor in the presence of oxygen to form a combustion gas stream and heat, and transferring at least a portion of the heat formed to the reforming reaction zone, the hydrocarbon stream, the water stream, or the retentate recycle stream, or combinations thereof.
The system of the present invention includes a reforming reactor having an inlet, an outlet, a reforming reaction zone, and a reforming catalyst located in the reforming reaction zone. The system also includes a hydrogen separating membrane for separating from the reformate stream a purified hydrogen stream and a retentate stream where the membrane has an inlet in flow communication with the outlet of the reforming reactor, a retentate side, and a permeate side. The system also includes a retentate recycle means for forming the retentate stream into a retentate recycle stream and an exhaust tail gas stream and for directing the retentate recycle stream to the reforming reaction zone. The system further includes a combustor having an inlet and an outlet, and capable of combusting the exhaust tail gas stream to generate heat and a combustion gas stream; and heat transfer means for transferring at least a portion of the heat formed in the combustor to the reforming reaction zone, the hydrocarbon stream, the water stream, or the retentate recycle stream or combinations thereof.
The present invention also provides a method of starting-up a reforming reactor used to supply hydrogen to a fuel cell. The method of starting-up the reforming reactor includes combusting a first portion of a hydrocarbon stream in the presence of an oxygen containing stream to generate heat and to form a starting combustion gas stream containing water vapor; feeding at least a portion of the starting combustion gas stream and a second portion of a hydrocarbon stream into a reforming reaction zone of a reactor; and reacting the hydrocarbon stream, and the water vapor to form hydrogen, where at least a portion of the hydrocarbon stream is vaporized from the heat generated in the combustion. The method also includes heating the reaction zone to at least a temperature of 200xc2x0 C. and a pressure of at least 100 kPa; and ceasing flow of the starting combustion gas stream into the reforming reaction zone.
In a preferred embodiment of the present invention a method and means is provided for using oxygen depleted air emitted from the fuel cell as an oxygen source for oxidizing the exhaust tail gas stream. Catalytic combustion when using this oxygen depleted air is particularly preferred.